A woman receiving an Ebola vaccination in Guinea in March. The clinical trial involved inoculating people who had come into contact with Ebola patients, and the contacts of those contacts.Credit
Cellou Binani/Agence France-Presse — Getty Images

An experimental Ebola vaccine being tested in the West African nation of Guinea during the continuing outbreak of the viral disease has shown promising initial results, according to a report on the clinical trial that was published Friday.

The report, in the British medical journal The Lancet, which analyzed 7,651 individuals, more than 3,500 of whom were vaccinated, indicated that the vaccine “might be highly efficacious and safe in preventing Ebola virus disease.”

“This is good news,” Dr. Margaret Chan, director general of the World Health Organization, said in an interview, while noting that the vaccine was not “a silver bullet” to head off future outbreaks. “There is no replacement for very strong and good, resilient health systems with the capability for surveillance,” she said.

Dr. Sakoba Keita, Guinea’s Ebola response coordinator and one of the study’s authors, said in an email that Guineans hoped the results would “contribute to the rapid control of the next epidemics caused by this disease.”

The clinical trial involved inoculating people who had come into contact with Ebola patients, and the contacts of those contacts. Some clusters of people were vaccinated immediately, while others, assigned randomly, were vaccinated only after 21 days, the maximum incubation period of the virus.

After 10 days, none of those who had been immediately vaccinated subsequently contracted Ebola, whereas 16 of the people eligible for the study who were not immediately vaccinated came down with the disease. The analysis was started at Day 10 to give the vaccine time to set off an immune response and because some of the contacts might already have been incubating the virus.

Those results were highly significant, the authors reported, indicating the vaccine was 75 to 100 percent efficacious. In fact, the researchers observed that there were no Ebola cases in any vaccinated patients after the first six days.

The results are “very encouraging,” said Dr. Anthony S. Fauci, the director of the National Institute of Allergy and Infectious Diseases, who was not involved in the trial, which he called a “very difficult study under very difficult circumstances.” He added that the study’s unorthodox design “makes it difficult to get the precise information you need,” but that, “even in the absence of that, the results are impressive.”

Another hope was that vaccinating many of those closest to an Ebola patient might quickly shut down a local outbreak, even among those not vaccinated. Despite tantalizing signs of effectiveness, the findings did not reach statistical significance, perhaps in part because roughly a third of those eligible for immediate vaccination did not consent to participate. In addition, the vaccine was not given to children or pregnant women because its safety in those populations was not yet known.

Because no placebos were used, volunteers knew whether they had received the vaccine, which could have influenced the study’s outcome. Choices like this, some of which were highly contentious on ethical and scientific grounds, were made to be pragmatic in the context of an emergency, said Jeremy Farrar, director of the Wellcome Trust, a major supporter of the research.

The fact that promising results were obtained in spite of certain compromises in scientific rigor, he said, “in a sense vindicates the design and approach that was taken and moves the field forward.”

Some outbreak responders initially opposed the idea of carrying out research in the midst of a dire emergency, but Dr. Farrar said this was imperative.

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This was the first time in memory that a vaccine’s effectiveness had been studied with a so-called ring vaccination approach, inoculating all of those around the occurrence of an infectious disease, according to one of the study’s senior authors, Dr. John-Arne Rottingen of the Norwegian Institute of Public Health. That vaccination strategy was used to help eradicate smallpox.

“I thought the only way to be successful would be to follow the epidemic and try to vaccinate in high-risk individuals,” Dr. Rottingen said in an interview. This approach ended up producing results even when the country had only a small number of cases, distributed across a large area.

The clinical trial in Guinea “has been more successful than we almost could have hoped for,” Dr. Rottingen said.

Other studies in Liberia and Sierra Leone, where the virus also raged, are continuing and are likely to produce results on safety and immune response but not on the effectiveness of the vaccine given the low numbers of patients in recent weeks, and the design, which did not target contacts of Ebola patients. The World Health Organization reported seven confirmed cases last week in the region, the lowest number of new Ebola patients in well over a year. On Friday, the United Nations announced the end of a special mission for Ebola emergency response.

Vaccinations began in March in Guinea. An independent safety monitoring board recommended that the interim results be published and that the design be altered to provide immediate vaccination to all eligible volunteers.

The study was led by the World Health Organization, the Guinean Health Ministry, Doctors Without Borders, the nonprofit Epicentre research center and the Norwegian Institute of Public Health, among many other contributing organizations. “This shows the power of partnerships,” said Dr. Marie-Paule Kieny, an assistant director general at the W.H.O. and the study’s other senior author.

Researchers working at the Public Health Agency of Canada created the experimental vaccine, rVSV-ZEBOV, which combines a piece of the Zaire Ebola virus’s covering with an animal virus, vesicular stomatitis virus, to set off an immune response against Ebola. It was licensed by NewLink Genetics, a biopharmaceutical company based in Ames, Iowa, in 2010, and then by Merck in November. The W.H.O. and its partners helped organize the study in Guinea after other studies supported by the United States government were planned for Liberia and Sierra Leone, and Guinea requested assistance to start its own. Other experimental Ebola vaccines are also being studied for safety.

The clinical trial included the use of a beer-keg-shaped vaccine storage device, the Arktek. It allowed the vaccines to be transported and kept at minus 80 degrees in outlying areas without electricity, which was deemed necessary because the vaccine’s stability at higher temperatures had not yet been fully tested. The storage device was invented by Global Good, a collaboration between the investment company Intellectual Ventures and Bill Gates.

A World Health Organization committee is expected to consider the results at a meeting next month and begin making recommendations for future use.

Experts familiar with the study said they expected the vaccine would be used for now only to respond to new outbreaks rather than to vaccinate entire populations, because it was not yet known how long protection would last. Doctors Without Borders is recommending that the study be extended so that vaccine can be given to all front-line workers and the contacts of all new cases in the region, said Dr. Bart Janssens, the group’s director of operations.

The next step will be submitting the data from this study and several others, including safety results, to regulatory bodies such as the United States Food and Drug Administration. Then the vaccine can be considered for licensure.

Dr. Mark Feinberg, chief public health and scientific officer at Merck Vaccines, said the company was committed to working with regulatory authorities toward licensure. “In the past, I think there wasn’t a strong belief that we needed an Ebola vaccine or that it would actually be possible to develop an Ebola vaccine,” Dr. Feinberg said. “Now I think there’s unanimity.”

A version of this article appears in print on August 1, 2015, on Page A9 of the New York edition with the headline: Ebola Vaccine Tested During Outbreak in Guinea Shows Promise, Report Says. Order Reprints|Today's Paper|Subscribe